A procedure was developed and verified as a viable means for collecting hydrocarbons in the diesel fuel range. The method was then used in a series of diesel and gasoline powered passenger car studies aimed at comparing the gas phase with the particle-bound hydrocarbon mutagenic activity. The diesel studies showed that the gas phase activity was less than 11% that of the particle-bound with this activity relation depending upon the test cycle. The diesel studies consisted of testing a 1978 Oldsmobile, a 220C Datsun, and a VW Rabbit. In the gasoline studies a 1972 Chevrolet Impala, a 1981 Dodge van, and a 1970 Ford van were tested. The Impala and Dodge were run on unleaded fuel and the Ford on commercially available leaded. The Dodge van had a light duty catalyst control system. The gasoline studies showed that the gaseous hydrocarbon activity to be at or near background level. A comparison of the particle-bound activity of the diesel and gasoline engines indicated the gasoline activity, in reverents/mile emissions, to be less than 12% that of the diesel.